As shown in FIG. 1, a prior editing system comprises: a VTR-A (1) for performing playback; a VTR-B (2), for recording a video signal 1a from VTR-A (1); a synchronization signal generator means 3 for synchronizing the signal processing systems of VTR-A (1) and VTR-B (2); a control means 4 for synchronously driving and controlling the record/playback operation of VTR-A (1) and VTR-B (2), respectively, based on editing time code TA played back by VTR-A (1) and editing time code TB played back by VTR-B (2); and a display means 5 for monitoring editing results.
The operation of the system will be described for the case in which, as shown in FIG. 2, the nth through (n+m)th frames of a video signal 1a being played back by VTR-A (1) from a magnetic tape on which it was previously recorded, is insertion-recorded (insertion-edited) onto a previously recorded magnetic tape loaded in VTR-B (2). In this particular system, the magnetic tapes are housed in cassettes which are loaded in the VTRs.
VTR-A (1) and VTR-B (2) are driven in frame synchronization with the video signal 1a, based on synchronization signals 3a and 3b generated by the synchronization signal generator means 3. That is, the tape transport and signal transfer systems are synchronized between VTR-A (1) and VTR-B (2). The video signal 2a monitored by display means 5 can be the playback or the record video signal from either VTR-A (1) or VTR-B (2), respectively. As an alternative, two display means 5 may be provided (one for each VTR).
In order to perform professional editing with VTRs, the time, that is, positional relationship between the record and the playback video signals on the magnetic tape must be precisely controlled. To accomplish this, time codes are recorded on the magnetic tapes on which the video signals are recorded. One way of doing this is to record the time code in a prescribed track running length on the tape or to insert the time code in the vertical blanking period of the video signal. The location at which recording is to start, and the location at which playback is to start, and so forth, can then be specified based on this time code.
Referring now to FIG. 2, assume that the video between the nth and (n+m)th frames of the signal being played back by VTR-A (2) is to be inserted in the segment between the Nth and (N+M)th frames of the video previously recorded on the tape loaded in VTR-B (2). When the nth frame of video is output by VTR-A (1), the control means 4 causes VTR-B (2) to start recording the video portion to be inserted, and when the (n+m)th frame of video is output by VTR-A (1), the control means 4 causes VTR-B (2) to stop recording. Assume that an input means (not illustrated) sends the control means 4 its instructions on where to start and stop recording.
It takes time for the playback signal picked up by the playback magnetic head of VTR-A (1) to pass through the playback signal processing system VTR-A (1) and be input to VTR-3 (2) as video signal 1a. It also takes time for this video signal 1a to pass through the record signal processing system of VTR-B (2) and be applied to its record magnetic head. If we assume that this processing time is small enough to be negligible for all practical purposes, the above insertion editing process can be performed with no problem.
In analog VTRs, it takes on the order of a few microseconds to perform the signal processing required to process an input video signal for recording by the magnetic head. It similarly takes only a few microseconds to process a recorded playback signal picked up by the magnetic head for output as a reproduced video signal. In practice, then, with analog VTRs, the above insertion editing can be performed without problems.
In a digital VTR, however, the video signal is converted to a digital data signal, which then undergoes data compression prior to recording. The digital video signal must therefore be decompressed when it is played back. When insertion editing is performed using digital VTRs, the signal processing required to record the signal (data compression, error-correction coding, shuffling, etc.) takes more than 1/30th of a second (the length of a video frame). The complementary processes on the playback end take just as long.
To perform insertion editing with digital VTRs as described above, for example, it would take two frame times for VTR-A (1) to output, process, and playback a signal 1a and two additional frame times for VTR-B (2) to input, process, and record it, for a total of four frame times.
FIG. 3 shows the recorded states of the V-B tape before and after editing. If editing were performed as described above, with the nth frame of the VTR-A (1) playback video used to time the start of the edit, frames (n-2) through (n+m-1) of the VTR-A (1) video would actually end up being recorded over frames (N+2) through (N+m+2) of the pre-edit VTR-B (2) video, as shown in FIG. 3. In other words, the inserted video will start on the edited tape two frames late, and its content will be delayed by four frames.
The following three methods might be considered as ways of correcting this editing offset.
(1) Set the timing of the VTR-A (1) output two frames ahead, and advance VTR-B (2) playback output timing ahead of the VTR-A (1) time code by two frames.
(2) Advance the VTR-A (1) time code output four frames ahead of the video output, and delay playback/record operations during editing (via the instructions provided to the control means by the user, for example) by two frames relative to the actual playback/record operation.
(3) In the VTRs, use separate record and playback heads mounted on the rotating drum so that the tape reaches the playback head four frames before it reaches the record head, and delay playback/record operations during editing by two frames relative to the actual playback/record operation.
With the above methods (1) and (2), however, there is a problem in that if digital VTRs with no recording or playback delays and digital VTRs with different delay times are mixed in the system, the time code timing and record/playback timing would both have to be adjusted to within precise tolerances, complicating the operation of the system.
With method (3), if, as shown in FIG. 4, the playback head were positioned ahead of the record head by from one track to a few tracks, for example, since there is not much distance between the points at which the record and playback heads first come in contact with the edge of the magnetic tape (the distance between Y1 and Y2 in FIG. 4), the playback head would be able to reproduce video from the leading track. Point 41, where the playback head first comes in contact with the edge of the magnetic tape, is actually inside the track, as shown in FIG. 4. The shaded part is hereafter identified as a preamble portion 42. This preamble portion 42 is recorded ahead of the digital signal area and is used to generate a clock signal required to reproduce the digital signal. Accordingly, there will be no problem as long as the playback head can start picking up data from the preamble portion 42. If, however, the playback head is positioned more than a few tracks ahead of the record head, as shown in FIG. 4, then point 43, at which the playback head now first comes in contact with the magnetic tap, would already be in the portion of the tape in which the digital signal is recorded. There would then be a problem in that it would not be possible to reproduce the entire digital signal. Also, since the preamble portion 42 could not be recovered, it would not be possible to generate the clock signal properly.